What does softened water mean?
Basically hard water contains minerals like calcium, magnesium and other cations. When it is sent through a softener, these hard mineral are replaced with sodium.
So soft water = more sodium content in water
When you soften water, you are simply replacing hard minerals with sodium.
Yes it is extremely harmful for your health.
There are no benefits to be had from drinking softened water. Some people claim that drinking soft water would increase the essential sodium content in your body. This reasoning is not founded in any logic because the human body cannot digest or assimilate the sodium present in water, just like it can't assimilate table salt.
Remember that human body is a living object and it cannot assimilate anything which comes from an inorganic source. The sodium from water is a waste to the body and it will be stored or disposed as toxin.
The human body does require sodium as it is an essential mineral for metabolism and nerve functioning. But what is the source of sodium?
It's not from table salt or from drinking water that the body assimilates useful sodium. The source of sodium is your food. Fruits, vegetables and seeds contain the required sodium content which the body absorbs through digestion.
The sodium acquired from table salt and from drinking softened water is just toxic to the body. It is stored in the body to be disposed with time. These salts are kept in a diluted state to keep them from harming the living cells.
If the quantity of inorganic sodium increases in your body, it will result in several malfunctions including high blood pressure.
Even when you cook using softened water you are increasing the inorganic sodium content in your food. It's just like adding more table salt. Remember that table salt or sodium taken externally in its inorganic form is extremely harmful to the body.